1. Field of the Invention
The present invention relates to a semiconductor device of a nitride-based compound semiconductor and a method for fabricating the semiconductor device. More particularly, the present invention relates to a semiconductor laser device emitting blue/violet light having a wavelength of around 400 nm.
2. Description of the Related Art
A nitride-based compound semiconductor made of gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), or a mixed crystal composed thereof has wide bandgap energy in a range of 1.9-6.2 eV, and is thus expected to be useful as a semiconductor material for a light-emitting or light-receiving device covering a range from visible light to ultraviolet light. In particular, a semiconductor laser device emitting a light having a wavelength of around 400 nm, such as that realized using this material, is expected to be a very feasible light source for a next-generation hyper-density optical disk, and the research and development thereof has been vigorously conducted throughout the world.
The practical use of such a semiconductor laser device as a light source for an optical disk critically requires the precision and uniformity of: thicknesses of a plurality of semiconductor layers included in the semiconductor laser device; and a structure of a waveguide, such as a buried structure, for achieving the single transverse mode oscillation. For a semiconductor laser device of a nitride-based compound semiconductor, in particular, it is important to use an etching technique which can control formation of the waveguide structure precisely and uniformly.
Some of conventional etching techniques for the nitride-based compound semiconductor will be described below.
One of the techniques is a dry etching technique using boron chloride (BCl3) and nitrogen (N2) as etching gases (F. Ren et al., Journal of Electronic Materials, Vol. 26, No. 11, 1997, pp. 1287-1291).
Another technique is an etching technique, so-called wet etching, in which a carrier-doped nitride-based compound semiconductor is etched by immersion in an aqueous solution of potassium hydroxide or phosphoric acid and illumination with light of larger energy than the bandgap energy of the nitride-based compound semiconductor (Japanese Laid-Open Publication No. 9-232681; C. Youtsey et al., Applied Physics Letters, Vol. 72, No. 5, 1998, pp. 560-562; and L.-H. Peng et al., Applied Physics Letters, Vol. 72, No. 8, 1998, pp. 939-941).
A nitride-based compound semiconductor laser device capable of obtaining the single transverse mode oscillation, which is fabricated using such techniques, is, for example, disclosed in Japanese Laid-Open Publication No. 6-19801. This semiconductor laser device will be described with reference to FIG. 5.
Referring to FIG. 5, the nitride-based compound semiconductor laser device includes a substrate 1, and further includes a buffer layer 2 of undoped GaN, an n-type contact layer 3 of n-type GaN, an n-type cladding layer 4 of n-type Al0.1Ga0.9N, an n-type light guiding layer 5 of n-type GaN, an active layer 6 being a multi-quantum-well layer produced by alternative formation of an In0.15Ga0.85N well layer and an In0.02Ga0.98N barrier layer, a p-type light guiding layer 7 of p-type GaN, and a first p-type cladding layer 8 of p-type Al0.1Ga0.9N, which are successively formed on the substrate 1. The nitride-based compound semiconductor laser device still further includes a groove structure 9 formed on the cladding layer 8, and a p-type contact layer 10 of p-type GaN formed on the groove structure 9. The groove structure 9 serves as part of a waveguide.
The groove structure 9 includes an n-type current blocking layer 12 of n-type Al0.1Ga0.9N in which a groove stripe is formed, and a second p-type cladding layer 11 of p-type Al0.5Ga0.95N formed on the current blocking layer 12.
A portion of a surface of the n-type contact layer 3 is exposed, and an n-type electrode 13 is formed on the exposed surface of the n-type contact layer 3. Also, a p-type electrode 14 is formed on the p-type contact layer 10.
A method of producing the groove structure 9 will now be described with reference to FIGS. 6A through 6C.
After the first p-type cladding layer 8 has been formed over the substrate 1, the n-type current blocking layer 12 is formed on the first p-type cladding layer 8. A mask 15 having an opening in the shape of a stripe with a predetermined width is attached on the n-type current blocking layer 12 (FIG. 6A).
Then, a portion of the n-type current blocking layer 12 corresponding to the opening of the mask 15 is removed by etching to form a groove (FIG. 6B). Subsequently, the mask 15 is removed. The second p-type cladding layer 11 is formed on the groove and the remaining n-type current blocking layer 12 (FIG. 6C).
The above-described conventional techniques have the following problems.
To provide satisfactory manufacturing yields of the nitride-based compound semiconductor laser device capable of the single transverse mode oscillation, it is critically important to control the shape and thickness of the groove structure 9. To achieve this end, it is necessary to precisely and uniformly control etching so as to obtain a precise and uniform depth of the groove resulting from the etching removal of the portion of the n-type current blocking layer 12 corresponding to the opening of the mask 15.
For conventional wet etching techniques, the above-mentioned reference discloses a technique in which an etch stop layer is used to stop etching at the etch stop layer. In this case, however, a layer to be etched and the etch stop layer are produced by changing the carrier densities thereof, although it is difficult to precisely and uniformly control the carrier densities. Therefore, the etching selectivity of the layer to be etched and the etch stop layer varies, which causes roughness on the etched surfaces of the layer to be etched and the etch stop layer.
For conventional dry etching techniques, an etching selectivity has not ben known with respect to the nitride-based compound semiconductor, so that it is not believed that an etch stop layer may be provided. In general, as the n-type current blocking layer 12 and the first p-type cladding layer 8 have the same or similar composition, etching does not stop at the n-type current blocking layer 12 and proceeds in the first p-type cladding layer 8. Accordingly, in the step of removing the n-type current blocking layer 12, it is difficult to precisely and uniformly control the etching depth.
Thus, it is difficult to provide satisfactory manufacturing yields of the nitride-based compound semiconductor laser device capable of the single transverse mode oscillation.
A semiconductor device of the present invention includes: a substrate; a multi-layer structure provided on the substrate; a first-conductive-type etch stop layer of a III nitride provided on the multi-layer structure; and a second-conductive-type first semiconductor layer of a III nitride provided on the etch stop layer. A molar fraction of Al is lower in a composition of the III nitride included in the first semiconductor layer than in a composition of the III nitride included in the etch stop layer.
Accordingly, since the etch stop layer having a larger molar fraction of Al in the composition thereof than the first semiconductor layer is formed immediately under the first semiconductor layer, etching performed in the first semiconductor layer is substantially stopped at the etch stop layer.
In one embodiment of the present invention, the multi-layer structure includes a second semiconductor layer of a III nitride, a molar fraction of Al being lower in a composition of the III nitride included in the second semiconductor layer than in a composition of the III nitride included in the etch stop layer.
Accordingly, the second semiconductor layer has a higher refractive index than the etch stop layer.
In one embodiment of the present invention, the multi-layer structure includes an active layer.
Accordingly, a semiconductor device having a highly precise waveguide structure can be obtained.
In one embodiment of the present invention, the first semiconductor layer does not allow electrical current to pass therethrough, and a groove is provided along the  less than 1, 1, xe2x88x922, 0 greater than  direction in the first semiconductor layer, the groove reaching the etch stop layer.
Accordingly, the first semiconductor layer functions as a current blocking layer, thereby obtaining satisfactory crystal growth on the sloped sides of the groove.
In one embodiment of the present invention, in the semiconductor device, the second conductive-type may be different from the first conductive-type.
A method for fabricating a semiconductor device according to the present invention includes the steps of: forming a multi-layer structure on a substrates forming a first-conductive-type etch stop layer of a III nitride on the multi-layer structure; forming a second-conductive-type first semiconductor layer of a III nitride on the etch stop layer, a molar fraction of Al being lower in a composition of the III nitride included in the first semiconductor layer than in a composition of the III nitride included in the etch stop layer; and selectively removing at least a portion of the first semiconductor layer by vapor etching.
Accordingly, a difference in an Al molar fraction between the etch stop layer and the first semiconductor layer causes the etching selectivity, thereby making it possible to precisely and uniformly control the etching of the first semiconductor layer.
In one embodiment of the present invention, the vapor etching is performed using a gas mixture of boron chloride and nitrogen.
Accordingly, since the gas mixture of boron chloride and nitrogen is used in dry etching, the uniformity of etching can be further improved.
In one embodiment of the present invention, the vapor etching provides a groove along the  less than 1, 1, xe2x88x922, 0 greater than  direction in the first semiconductor layer.
Accordingly, satisfactory crystal growth can be obtained on the sloped sides of the groove provided along the  less than 1, 1, xe2x88x922, 0 greater than  direction in the first semiconductor layer.
In one embodiment of the present invention, in the method, the second conductive-type may be different from the first conductive-type.
Thus, the invention described herein makes possible the advantages of (1) providing a semiconductor laser having a more precise waveguide structure than the prior art; and (2) providing a method for fabricating such a semiconductor device.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.